In order to reduce harmful gas components discharged from,fuel-injection spark-ignition internal combustion engines, which are mounted in automobiles and the like, or improve the fuel-efficiency of the engines, there have recently been proposed various engines of a cylinder-injection type (hereinafter referred to as cylinder-injection gasoline engines) in which the fuel is injected directly into the combustion chamber, in place of conventional manifold-injection engines.
A cylinder-injection gasoline engine is designed so that an air-fuel mixture with an air-fuel ratio close to the theoretical air-fuel ratio is fed locally into a space around a spark plug and a cavity in a piston, thereby enabling ignition with a generally fuel-lean air-fuel ratio. Accordingly, the deliveries of CO and HC are reduced, and the fuel-efficiency during idle operation or steady running of the engine is improved considerably. Moreover, the cylinder-injection gasoline engine can enjoy a much improved response to acceleration and deceleration, since it is free from a delay in fuel transportation through suction pipes in increasing or decreasing the fuel injection quantity. If the fuel injection quantity is increased so that the overall air-fuel ratio (or average air-fuel ratio) comes closer to the theoretical air-fuel ratio as the load increases, however, an excessively fuel-rich state is established in the vicinity of the spark plug, so that the so-called rich misfire takes place. Thus, it is difficult to keep the local air-fuel ratio in the vicinity of the spark plug at an optimum value throughout the entire engine operation region to maintain the stability of the engine operation.
In order to eliminate these drawbacks, a cylinder-injection engine is proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 5-79370, which is arranged such that the fuel is injected with an appropriate timing depending on the engine load and that the configuration of the combustion chamber is designed to match such a fuel injection method. In this proposed engine, the injection mode is changed in accordance with the load, between a second-term injection mode for injecting the fuel in a compression stroke and a first-term injection mode for injecting the fuel in a suction stroke. Specifically, during low-load operation, the fuel is injected into the cavity in the final stage of the compression stroke, whereby the air-fuel mixture with the air-fuel ratio close to the theoretical air-fuel ratio is formed locally around the spark plug and in the cavity. As a result, ignition even with a generally lean air-fuel ratio is enabled, the deliveries of CO and HC are reduced, and the fuel-efficiency for idle operation or normal drive is improved considerably. During medium-load operation, the first-term injection mode is selected to inject the fuel into the cavity in the initial stage of the suction stroke, whereby the air-fuel mixture is collected in the cavity to stabilize the combustion. In this case, a relatively rich air-fuel mixture is formed around the spark plug by injecting a small quantity of fuel into the cavity in the latter half of the compression stroke, thereby further stabilizing the ignition and combustion. During high-load operation, on the other hand, the fuel is injected outside the cavity in the suction stroke so that an air-fuel mixture with a uniform air-fuel ratio is formed in the combustion chamber. As a result, the fuel can be burned in a quantity equal to that for manifold-injection gasoline engines, and a required engine output for starting-accelerating operation can be secured.
According to the proposed cylinder-injection gasoline engine described above, the overall air-fuel ratio can be set at a very large value in the secondterm injection mode. Accordingly, lean combustion for low-load operation, such as idling, can be enabled by supplying plenty of suction air through a passage bypassing a throttle valve or by recirculating exhaust gas (hereinafter referred to as EGR) in abundance, so that the deliveries of harmful gas components can be reduced, and the fuel-efficiency can be improved.
In setting the overall air-fuel ratio for the second-term injection mode at a very large value (e.g., 22 to 40) in order to improve the exhaust gas characteristics and fuel-efficiency, however, the value of the air-fuel ratio on the rich side of the engine operation region is restricted to about 20 to 22. If the overall air-fuel ratio is more fuel-rich than this limit value, a rich misfire or smoke may be caused. If an attempt is made to change the control mode from the second-term injection mode, which is subject to such a restrictive condition on the air-fuel ratio, into the first-term injection mode, in which uniform-mixture combustion best suited for acceleration is carried out, the air-fuel ratio changes discontinuously during the mode change, thereby causing a shock and ruining the drivability.
In changing the mode from the second-term injection mode, in which plenty of bypass air and EGR is introduced, into the first-term injection mode, moreover, it is necessary to control the bypass air quantity and EGR value, as well as the fuel injection quantity, injection timing, ignition timing, etc. Thus, it is very difficult smoothly to make all changes (transitions) between the modes in various engine operation states.